Capacitive sensing system able of using heating element as antenna electrode

ABSTRACT

A capacitive sensing system for being connected to a heating element comprises a capacitive detector connectable to the heating element and a common mode choke for essentially preventing alternating current from flowing from the heating element to the heating current supply. The capacitive detector is configured for driving an alternating current into the heating element and for producing an output indicative of capacitance based upon the alternating current. The choke has a first and a second winding for connecting the heating element with the heating current supply. The choke comprises a third winding connected in parallel of the first and/or second winding. The capacitive detector is configured for measuring a portion of the alternating current flowing across the third winding and for taking into account the measured portion of alternating current when producing the output.

TECHNICAL FIELD

The present invention generally relates to a capacitive sensing systemthat can use a heating element as its antenna electrode. A capacitivesensing system as proposed herein may e.g. be used for detecting theabsence or presence of an occupant on an occupiable item, which might bea vehicle seat, a hospital bed etc., or for detecting a driver's hand onthe steering wheel.

BACKGROUND ART

A capacitive sensor, called by some electric field sensor or proximitysensor, designates a sensor, which generates a signal responsive to theinfluence of what is being sensed (a person, a part of a person's body,a pet, an object, etc.) upon an electric field. A capacitive sensorgenerally comprises at least one antenna electrode, to which is appliedan oscillating electric signal and which thereupon emits an electricfield into a region of space proximate to the antenna electrode, whilethe sensor is operating. The sensor comprises at least one sensingelectrode at which the influence of an object or living being on theelectric field is detected. In some (so-called “loading mode”)capacitive occupancy sensors, the one or more antenna electrodes serveat the same time as sensing electrodes. In this case, the measurementcircuit determines the current flowing into the one or more antennaelectrodes in response to an oscillating voltage being applied to them.The relationship of voltage to current yields the complex impedance ofthe one or more antenna electrodes. In an alternative version ofcapacitive sensors (“coupling mode” capacitive sensors), thetransmitting antenna electrode(s) and the sensing electrode(s) areseparate from one another. In this case, the measurement circuitdetermines the current or voltage that is induced in the sensingelectrode when the transmitting antenna electrode is operating.

The different capacitive sensing mechanisms are explained in thetechnical paper entitled “Electric Field Sensing for GraphicalInterfaces” by J. R. Smith, published in Computer Graphics I/O Devices,Issue May/June 1998, pp 54-60. The paper describes the concept ofelectric field sensing as used for making non-contact three-dimensionalposition measurements, and more particularly for sensing the position ofa human hand for purposes of providing three dimensional positionalinputs to a computer. Within the general concept of capacitive sensing,the author distinguishes between distinct mechanisms he refers to as“loading mode”, “shunt mode”, and “transmit mode” which correspond tovarious possible electric current pathways. In the “loading mode”, anoscillating voltage signal is applied to a transmit electrode, whichbuilds up an oscillating electric field to ground. The object to besensed modifies the capacitance between the transmit electrode andground. In the “shunt mode”, an oscillating voltage signal is applied tothe transmit electrode, building up an electric field to a receiveelectrode, and the displacement current induced at the receive electrodeis measured, whereby the displacement current may be modified by thebody being sensed. In the “transmit mode”, the transmit electrode is putin contact with the user's body, which then becomes a transmitterrelative to a receiver, either by direct electrical connection or viacapacitive coupling. “Shunt mode” is alternatively referred to as theabove-mentioned “coupling mode”.

Capacitive occupant sensing systems have been proposed in great variety,e.g. for controlling the deployment of one or more airbags, such as e.g.a driver airbag, a passenger airbag and/or a side airbag. U.S. Pat. No.6,161,070, to Jinno et al., relates to a passenger detection systemincluding a single antenna electrode mounted on a surface of a passengerseat in an automobile. An oscillator applies on oscillating voltagesignal to the antenna electrode, whereby a minute electric field isproduced around the antenna electrode. Jinno proposes detecting thepresence or absence of a passenger in the seat based on the amplitudeand the phase of the current flowing to the antenna electrode. U.S. Pat.No. 6,392,542, to Stanley, teaches an electric field sensor comprisingan electrode mountable within a seat and operatively coupled to asensing circuit, which applies to the electrode an oscillating or pulsedsignal “at most weakly responsive” to wetness of the seat. Stanleyproposes to measure phase and amplitude of the current flowing to theelectrode to detect an occupied or an empty seat and to compensate forseat wetness.

The idea of using the heating element of a seat heater as an antennaelectrode of a capacitive occupancy sensing system has been known for along time. WO 92/17344 A1 discloses a an electrically heated vehicleseat with a conductor, which can be heated by the passage of electricalcurrent, located in the seating surface, wherein the conductor alsoforms one electrode of a two-electrode seat occupancy sensor.

WO 95/13204 discloses a similar system, in which the oscillationfrequency of an oscillator connected to the heating element is measuredto derive the occupancy state of the vehicle seat.

U.S. Pat. No. 7,521,940 relates to a combined seat heater and capacitivesensor capable of operating, at a time, either in heating mode or inoccupant-sensing mode. The device includes a sensor/heat pad fortransmitting a sensing signal, a first diode coupled to a first node ofthe sensor/heat pad, a second diode coupled to a second node of thesensor/heat pad, a first transistor coupled to the first diode and asecond transistor coupled to the second diode. During sensing mode, thefirst and second transistors are opened and the nodes between the firsttransistor and the first diode, as well as between the second transistorand the second diode are reverse-biased to isolate the sensor/heat padfrom the power supply of the heating circuit.

U.S. 2009/0295199 discloses a combined seat heater and capacitivesensor, wherein each of the two terminals of the heating element isconnected to the heating power supply via two transistors in series. Thedevice may not operate in sensing mode and in heating mode at a time.When the device is in sensing mode, the nodes between each pair oftransistors are actively kept at the same potential as the heatingelement by means of respective voltage followers in order to neutralizeany open-switch impedance of the transistors.

The very same idea has already been disclosed in U.S. Pat. No.6,703,845. As an alternative to transistors, that document disclosesinductors to achieve a high impedance at the frequency of theoscillating signal between the heating element and the power source ofthe heating circuit. As in the previously discussed document, a voltagefollower maintains the intermediate nodes substantially at the samepotential as the heating element in order to effectively isolate, at thefrequency of the oscillating signal, the power supply of the heatingcircuit from the heating element.

A disadvantage of the system disclosed in U.S. Pat. No. 6,703,845 isthat the inductors used as AC-decoupling elements have to support thefull heating current (up to 10 A DC and more) and present high ACimpedance to the capacitive measurement circuit and the seat heater atthe same time. High inductance and high operating DC current impliesthat the inductor have to be wound on large cores, which are expensive.Depending on the application chosen from U.S. Pat. No. 6,703,845, eithertwo or four of these inductors have to be used.

U.S. 2011/121618 discloses yet another variant of an occupant detectionsystem including a heating element adjacent the seating surface of aseat. A capacitive occupant detection circuit is electrically coupled tothe heating element. A common mode choke is interposed between theheating circuit and the heating element as an isolation circuit thatprevents the heating circuit from influencing the occupant detectioncircuit.

An ideal common mode choke would totally prevent flow of an AC current.In practice, however, an ideal common mode choke does not exist andthere will be a residual alternating current across the common modechoke. In order to reduce this residual alternating current to an extentthat the isolation circuit can be considered to prevent the heatingcircuit from influencing the occupant detection circuit, i.e. to anextent that the residual alternating current can be neglected incomparison to the alternating current flowing across the capacitance tobe measured, one would have to choose (among the existing common modechokes) a common mode choke whose resistance to the heating currentwould result in a considerable loss of heating power in the common modechoke.

BRIEF SUMMARY

An improved capacitive sensing system is provided that can use a heatingelement as an antenna electrode.

A capacitive sensing system for being connected to a heating elementproducing heat upon electrical current being caused to flow there acrosscomprises a capacitive detector connectable to the heating element and acommon mode choke for essentially preventing alternating current fromflowing from the heating element to the heating current supply. Thecapacitive detector is configured for driving an alternating currentinto the heating element and for producing an output indicative ofcapacitance based upon the alternating current. The common mode chokehas a first winding for connecting a first terminal of the heatingelement with a first terminal of the heating current supply and a secondwinding for connecting a second terminal of the heating element with asecond terminal of the heating current supply. According to an aspect ofthe invention, the common mode choke comprises a third winding (wound onthe same core as the first and second windings) connected in parallel ofat least one of the first and second windings and the capacitivedetector is further configured for measuring a portion of thealternating current flowing across the third winding and for taking intoaccount the measured portion of alternating current when producing theoutput.

The present invention uses a common mode choke with a coupling factorclose to unity to achieve AC-decoupling of the heating element from itspower supply. The common mode choke produces high impedance againstcommon mode currents from the heating element into the first and secondwindings. The capacitive sensing system according to the inventionfurthermore takes into account the fact that an actual common mode chokecannot perfectly block alternating current. The total residualalternating current across the common mode choke (i.e. the part thatcannot be blocked) stands in a known relationship to the portion of thealternating current that flows across the third winding. By measuringthis portion of the alternating current, the capacitive sensing systemthus can determine the alternating current actually flowing across thecapacitance of the heating element.

The capacitive sensing system as described above may be configured as amodule so as to be easily connectable between to an existing heatingelement and its heating current supply. A preferred alternative aspectof the invention concerns a combined (integrated) heating and capacitivesensing system. Such a combined system comprises a heating element forproducing heat when electrical current is caused to flow across it, aheating current supply having a first terminal connected to a firstterminal of the heating element and a second terminal connected to asecond terminal of the heating element so as to form a heating circuit,and a capacitive detector connected to the heating element, configuredfor driving an alternating current into the heating element and forproducing an output indicative of capacitance based upon the alternatingcurrent. A common mode choke is provided for essentially preventing thealternating current from flowing from the heating element to the heatingcurrent supply. The common mode choke has a first winding connectedbetween the first terminal of the heating element and the first terminalof the heating current supply and a second winding connected between thesecond terminal of the heating element and the second terminal of theheating current supply. The common mode choke comprises a third windingconnected in parallel of at least one of the first and second windings.The capacitive detector is further configured for measuring a portion ofthe alternating current flowing across the third winding and for takinginto account the measured portion of alternating current when producingthe output.

The combined heating and capacitive sensing system could e.g. be used ina steering wheel and/or a vehicle seat.

The combined heating and capacitive sensing system may be implemented asa combined seat heating and capacitive occupancy sensing system, theoutput produced by the capacitive detector being in this case indicativeof a seat occupancy state.

In the following, it will be assumed that the heating current is directcurrent (DC) and that the capacitive measurement uses alternatingcurrent (AC) at a certain frequency. This is insofar a simplificationthat transient states (e.g. switching on/or off of the heating current),noise and parasitic currents are not taken into account. It should alsobe noted that the heating current need not be direct current in thestrictest sense: it may be variable, but on a long time-scale, so as notto interfere with the current used for the capacitive measurement. Forsake of simplicity, we will use “DC” to designate slowly varying orconstant signals. The capacitance measurement network preferablyoperates at frequency selected in the range from about 50 kHz to about10 GHz, more preferably in the range from about 50 kHz to about 30 MHz.

Preferably, the capacitive detector comprises a first current meterconfigured and arranged for measuring the alternating current driveninto the heating element.

Preferably, the capacitive detector comprises a second current meterconfigured and arranged for measuring the portion of the alternatingcurrent flowing across the third winding.

It follows from Kirchhoff's junction rule that the alternating currentdissipated via the heating element can be calculated by withdrawing thetotal alternating current across the common mode choke from thealternating current driven into the heating element. The capacitivedetector thus preferably comprises an evaluation circuit operativelyconnected to the mentioned current meters, the evaluation circuit beingconfigured for producing the output indicative of the capacitance usingthe measurement signals of the current meters.

According to a preferred embodiment of the invention, the third windingis AC-coupled (e.g. via a coupling capacitor) to a reference node, e.g.circuit ground.

As indicated above, the portion of alternating current flowing acrossthe third winding is indicative of the alternating current flowingacross the common mode choke altogether. Assuming that the first, secondand third windings are identical and AC-coupled to the same node on theside opposite the heating element, the alternating currents across thewindings will be identical. The total alternating current I_(choke)across the common mode choke may in this case be calculated asI_(choke)=I₁+I₂+I₃=3·I₃, where I₁, I₂ and I₃ designates the alternatingcurrent across the first, second and third winding, respectively. On theside opposite the heating element, the first and second windings may beconnected to the reference node with a low AC-impedance (e.g. bycoupling capacitors). In order to avoid that the second current meterintroduces to high an AC impedance between the third winding and thereference node, a step-up transformer may be used. The step-uptransformer preferably has its primary winding connected between thethird winding and the reference node and its secondary winding connectedbetween the reference node and the second current meter. As indicated bythe term step-up transformer, the number of turns of the primary windingis lower than the number of turns of the secondary winding. Using thisconfiguration, the AC impedance caused by the second current meter willbe reduced by the turn ratio of the step-up transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of a combined heating andcapacitive sensing system according to a first example of the invention;

FIG. 2 is a schematic circuit diagram of a combined heating andcapacitive sensing system according to a second example of theinvention;

FIG. 3 is a schematic circuit diagram of a combined heating andcapacitive sensing system according to a third example of the invention;

FIG. 4 is a schematic representation of a vehicle seat equipped with acombined heating and capacitive sensing system.

FIG. 5 is a schematic representation of a steering wheel equipped with acombined heating and capacitive sensing system.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a combined heating and capacitive sensing system 10according to a first example of the invention. The combined system 10comprises a heating element 12 that produces heat when electricalcurrent is caused to flow across it. The heating element 12 may comprisea conductive wire, cable, fibre, bundle of fibres or a conductive track(e.g. made of a PTC material) printed on a flexible support. The heatingelement 12 has a first 14 and a second 16 terminal connected to a first18 and a second 20 terminal of a heating current supply 22,respectively. The heating current supply 22 (e.g. a direct currentsource and control electronics) and the heating element 12 form togetherthe heating circuit of a heater, e.g. for a vehicle seat. The terminalsof the heating current supply 22 typically have low AC impedance toAC-ground (e.g. due to coupling capacitors 23) in order to avoid that ACsignals disturb the heating current supply 22. The heating currentsupply preferably comprises a user-actuatable master switch (not shown)allowing the user to activate or deactivate the heating circuit as awhole and a temperature controller (not shown; including e.g. athermostat) that regulates the temperature to a comfortable level. Whenthe heater is operating, the temperature controller opens and closes theheating circuit (low-frequency pulse-width modulation of the heatingcurrent) in such a way as to achieve a preset target temperature.Preferably, the target temperature may be selected by the user using atemperature control interface (e.g. a knob, a slider, a wheel or thelike).

The combined heating and capacitive sensing system 10 further comprisesa capacitive detector 24 connected to the heating element 12. Thecapacitive detector 24 is configured for using the heating element 12 asits antenna electrode. In particular, during operation, the capacitivedetector 24 drives an alternating current into the heating element 12and produces an output indicative of the capacitance between the heatingelement 12 and AC-ground. based upon the alternating current. In FIG. 1,block 26 symbolically represents the capacitive coupling (the compleximpedance) of the heating element 12 to a grounded electrode (typicallythe vehicle frame). The complex impedance 26 between the heating element12 and the grounded electrode depends on whether the space between theheating element 12 and the grounded electrode is occupied by aconductive body or not. The capacitive detector 24 illustrated in FIG. 1operates in so-called loading mode. An oscillator 40 (e.g. an NCO) and afirst current meter 42 are connected in series to the heating element12.

The capacitive detector 24 comprises a common mode choke 28 with acoupling factor close to unity for essentially preventing thealternating current from flowing from the heating element 12 to theheating current supply 22. The common mode choke comprises threewindings 30, 32, 34 arranged on the same magnetic core (and having thesame number of windings). The first winding 30 is connected between thefirst terminal 14 of the heating element 12 and the first terminal 18 ofthe heating current supply 22. The second winding 32 is connectedbetween the second terminal 16 of the heating element 12 and the secondterminal 20 of the heating current supply 22. The third winding 34 isAC-coupled, via coupling capacitor 36 and current meter 38, toAC-ground.

During operation of the capacitive detector, the oscillator 40 appliesan alternating voltage, causing an alternating current I_(IN) to flowinto the heating element 12. Part of the current flows across impedance26 to AC-ground (current I_(X)). Since the common mode choke 28 cannotperfectly block alternating current, another part of the applied currentflows to AC-ground via the windings 30, 32 and 34 of the common modechoke 28 (current I_(choke)=I₁+I₂+I₃, where I₁, I₂ and I₃ designates thealternating current across the first, second and third winding,respectively). From Kirchhoff's junction rule: I_(IN)=I_(X)+I_(choke).Since I_(X) contains the information about impedance 26, it is notsufficient to measure I_(IN) if I_(choke) cannot be neglected. Asindicated above, it may be difficult, in practice, to design a commonmode choke that presents both sufficient impedance to the alternatingcurrent of the capacitive sensor and low resistance to the directheating current.

In accordance with the principles of the invention, in the illustratedexample, the capacitive detector 24, in particular its second currentmeter 38, measures the part of the alternating current flowing acrossthe third winding 34 of the common mode choke 28. Evaluation circuit 44is connected to the first 42 and second current 38 meter so as toreceive their measurement signals, indicative of I_(IN) and I₃,respectively. The AC impedance of the series connection of the couplingcapacitor 36 and the second current meter 38 is adjusted in such a waythat it is substantially equal to the AC impedance between the terminals18 and 20 of the heating current supply 22 and AC-ground. That way, thealternating currents I₁, I₂ and I₃across the three windings 30, 32 and34 of the common mode choke 28 are substantially equal in amplitude andphase. The evaluation circuit may thus calculate the total alternatingcurrent across the common mode choke 28 as I_(choke)=3·I₃. Thesubtraction of I_(choke) from I_(IN) yields I_(X). The multiplier 46carries out the multiplication of the measurement value of I₃ by −3. Theresult of the multiplication is added to the measurement value of I_(N)in adder 48, whereby the influence of the common mode choke isessentially compensated. The value of I_(X) thus obtained is fed todecision circuit 50, which outputs the value of the complex impedance 26and/or an occupancy status depending on I_(X).

FIG. 2 shows a combined heating and capacitive sensing system 10according to a second example of the invention. The only difference fromthe first example is that the decision circuit 50 of the evaluationcircuit is directly connected to the current meters 42 and 38. Thedecision circuit 50 in this example takes its decision based on the twomeasurement signals as inputs. In all other respects, the exampleillustrated in FIG. 2 is identical to the example illustrated in andexplained with respect to FIG. 1.

FIG. 3 shows a combined heating and capacitive sensing system 10according to a third example of the invention. The third example differsfrom the first example (FIG. 1) in that the capacitive detector 24comprises a step-up transformer 52 arranged between the third winding 34and the second current meter 38. The primary winding of the step-uptransformer 52 is connected between the third winding 34 of the commonmode choke 28 and AC-ground and the secondary winding of the step-uptransformer 52 is connected between AC-ground the input node of thesecond current meter 38. The number of turns N₁ of the primary windingis less than the number of turns N₂ of the secondary winding. Thestep-up transformer lowers the impedance “seen” by the alternatingcurrent across the third winding by a factor of about N₁/N₂.Accordingly, the configuration of FIG. 3 may be used if the secondcurrent meter 38 has too high input impedance compared to the impedancesbetween the terminals 18, 20 and AC-ground. It should be noted that thecurrent I₃′ input to the second current meter 38 amounts only toN₁/N₂·I₃. This has to be taken into account in the further calculations,e.g. in the multiplier 46. In this example, I_(X) can finally becalculated by the formula: I_(X)=I_(IN)−3·N₂/N₁·I₃′.

FIG. 4 is a schematic representation of a vehicle seat 54 equipped witha combined heating and capacitive sensing system 10 in accordance withthe invention, e.g. as illustrated in any one of FIGS. 1 to 3.

FIG. 5 is a schematic representation of a steering wheel 56 equippedwith a combined heating and capacitive sensing system 10 in accordancewith the invention, e.g. as illustrated in any one of FIGS. 1 to 3.

While specific embodiments have been described in detail, those skilledin the art will appreciate that various modifications and alternativesto those details could be developed in light of the overall teachings ofthe disclosure. Accordingly, the particular arrangements disclosed aremeant to be illustrative only and not limiting as to the scope of theinvention, which is to be given the full breadth of the appended claimsand any and all equivalents thereof.

What is claimed is:
 1. Capacitive sensing system for being connected toa heating element producing heat upon electrical current being caused toflow across it, said capacitive sensing system comprising a capacitivedetector connectable to said heating element, configured for driving analternating current into said heating element and for producing anoutput indicative of capacitance based upon said alternating current; acommon mode choke having a first winding for connecting a first terminalof said heating element with a first terminal of a heating currentsupply and a second winding for connecting a second terminal of saidheating element with a second terminal of said heating current supply,said common mode choke essentially preventing said alternating currentfrom flowing from said heating element to said heating current supply;wherein said common mode choke comprises a third winding connected inparallel of at least one of said first and second windings, saidcapacitive detector being further configured for measuring a portion ofsaid alternating current flowing across said third winding and fortaking into account said measured portion of alternating current whenproducing said output.
 2. Capacitive sensing system as claimed in claim1, wherein said capacitive detector comprises a first current meterconfigured and arranged for measuring said alternating current driveninto said heating element.
 3. Capacitive sensing system as claimed inclaim 2, wherein said capacitive detector comprises a second currentmeter configured and arranged for measuring said portion of saidalternating current flowing across said third winding.
 4. Capacitivesensing system as claimed in claim 3, wherein said capacitive detectorcomprises an evaluation circuit operatively connected to said currentmeters, said evaluation circuit being configured for producing saidoutput using measurement signals of said current meters.
 5. Capacitivesensing system as claimed in claim 1, wherein said third winding isAC-coupled to a reference node.
 6. Capacitive sensing system as claimedin claim 5, wherein said capacitive detector comprises a first currentmeter configured and arranged for measuring said alternating currentdriven into said heating element, wherein said capacitive detectorcomprises a second current meter configured and arranged for measuringsaid portion of said alternating current flowing across said thirdwinding, and wherein said capacitive sensing system comprises a step-uptransformer having a primary winding connected between said thirdwinding and said reference node and a secondary winding connectedbetween said reference node and said second current meter.
 7. Combinedheating and capacitive sensing system, comprising a heating element forproducing heat when electrical current is caused to flow across it, aheating current supply having a first terminal connected to a firstterminal of said heating element and a second terminal connected to asecond terminal of said heating element so as to form a heating circuit,a capacitive detector connected to said heating element, configured fordriving an alternating current into said heating element and forproducing an output indicative of capacitance based upon saidalternating current; a common mode choke having a first windingconnected between said first terminal of said heating element and saidfirst terminal of said heating current supply and a second windingconnected between said second terminal of said heating element and saidsecond terminal of said heating current supply, said common mode chokeessentially preventing said alternating current from flowing from saidheating element to said heating current supply; wherein said common modechoke comprises a third winding connected in parallel of at least one ofsaid first and second windings, said capacitive detector being furtherconfigured for measuring a portion of said alternating current flowingacross said third winding and for taking into account said measuredportion of alternating current when producing said output.
 8. Combinedheating and capacitive sensing system as claimed in claim 7, whereinsaid capacitive detector comprises a first current meter configured andarranged for measuring said alternating current driven into said heatingelement.
 9. Combined heating and capacitive sensing system as claimed inclaim 8, wherein said capacitive detector comprises a second currentmeter configured and arranged for measuring said portion of saidalternating current flowing across said third winding.
 10. Combinedheating and capacitive sensing system as claimed in claim 9, whereinsaid capacitive detector comprises an evaluation circuit operativelyconnected to said current meters, said evaluation circuit beingconfigured for producing said output using measurement signals of saidcurrent meters.
 11. Combined heating and capacitive sensing system asclaimed in claim 7, wherein said third winding is AC-coupled to areference node.
 12. Combined heating and capacitive sensing system asclaimed in claim 11, wherein said capacitive detector comprises a firstcurrent meter configured and arranged for measuring said alternatingcurrent driven into said heating element, wherein said capacitivedetector comprises a second current meter configured and arranged formeasuring said portion of said alternating current flowing across saidthird winding, and comprising a step-up transformer having a primarywinding connected between said third winding and said reference node anda secondary winding connected between said reference node and saidsecond current meter.
 13. Combined heating and capacitive sensing systemas claimed in claim 7, implemented as a combined seat heating andcapacitive occupancy sensing system, said output produced by saidcapacitive detector being indicative of a seat occupancy state. 14.Vehicle seat equipped with a combined heating and capacitive sensingsystem as claimed in claim
 13. 15. Steering wheel comprising a combinedheating and capacitive sensing system as claimed in claim 7.